The present invention relates to analogue to digital converters.
A number of different types of analogue to digital (A/D) converters are known. Most of the devices are fairly slow if high precision is required, as for example when `dual store` or `ramp` techniques are used. If high speed is required, `successive approximation` or `flash` converters are used at the expense of resolution.
Methods used to increase the resolution of the faster converters such as `sub-ranging` or introducing `dither` on the input signal have disadvantages. At present the best precision A/D converters available have approximately 14 bits accuracy at a conversion rate of 150 KHz. This means that the ultimate bandwidth of the system is about 50 KHz if aliasing is to be avoided.
Much higher resolutions and speeds are available using a known interpolating feedback system as shown in FIG. 1. Such known interpolating analogue to digital converters usually comprise a relatively coarse resolution analogue to digital (A/D) converter arranged in a feedback loop in combination with a digitial to analogue (D/A) converter; the D/A converter having much finer resolution than the coarse A/D converter. The coarse A/D converter, typically 6 bits resolution, is sampled at a high sample rate, typically 10 MHz, with subsequent averaging of the output samples obtained from the coarse resolution A/D converter.
The quantised signal appearing at the output of the coarse A/D converter is an approximation of the analogue signal fed to it, the approximation being dependent upon the quantum jump or step size of the quantisation and the sample rate used in the quantisation process. At any point in time, the difference between the analogue input signal and the output sample from the coarse A/D converter is known as the quantisation error; generally termed Q.
The output from the coarse A/D converter is fed to the finer resolution D/A converter, the output of which is fed back and combined with the incoming analogue input signal to be digitised. However the D/A converter utilised also gives rise to errors; generally termed T, the resolution of the D/A converter.
Such interpolating A/D converters also include an analogue controller, usually in the form of a high gain amplifier of gain A, in the feed path to the coarse A/D converter.
The digital output of this type of interpolating A/D converter can be expressed as ##EQU1## where D.sub.OUT is the digital output signal
V.sub.I is the analogue input signal PA1 Q is the quantisation error of the coarse resolution A/D converter PA1 T is the error in resolution of the D/A converter and PA1 A is the open loop gain of the analogue controller.
It can be seen from this expression that the term in the quantisation error Q is minimised by maximising the open loop gain A of the analogue controller. However, an increase in the open loop gain A does not improve the linearity of the converter, that is the relationship between the digital output and the analogue input, as the term in the resolution T of the D/A converter remains constant for all but small values of open loop gain A. Furthermore, by maximising the open loop gain A, it becomes extremely difficult to maintain loop stability in the converter. In practice, the resolution of such converters is limited by the value of the term in the quantisation error Q and in modern interpolating A/D converters is usually limited to about 14 bits.
Some disadvantages of the known interpolating feedback systems which have not yet been overcome to date include:
1. The ultimate dynamic range of the A/D converter is limited by the accuracy of the D/A converter used in its feedback path.
2. The complexity of an averaging filter at the output of the A/D converter prohibits the easy realisation of high resolution systems.
3. The design of the system is not optimised such that the clock rates used are usually excessive which results in difficult post processing.